1. Field of the Invention
This invention relates to a die casting machine and a die casting method that use a powder mold releasing agent as a mold releasing agent for easily releasing a cast from a mold.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 62-127150 discloses a die casting machine for conducting die casting by using a powder mold releasing agent. In this die casting machine, a mold comprising a fixed mold and a movable mold is clamped, and the inside of a mold cavity is evacuated through an exhaust port communicating with the cavity after this mold clamping. A mold releasing agent such as a powder mold releasing agent is supplied and applied into the mold cavity under the vacuum state through a sleeve.
The powder mold releasing agent provides various advantages in comparison with a liquid mold releasing agent. For example, when the liquid mold releasing agent is heated by a molten charge, the amount of heat decomposition gases is large, causing casting porosity in the die cast product. The powder mold releasing agent can decrease the occurrence of such casting porosity. The liquid mold releasing agent is generally sprayed, using an air flow, onto the mold surface. However, this method generates mist and noise and deteriorates the working environment. When the liquid mold releasing agent is sprayed, the temperature of the mold that is heated by the molten charge drops drastically, and the temperature change of the mold in one cycle of die casting becomes greater. As a result, life of the mold drops and hair line cracks, etc, occur at a relatively early stage.
In contrast, when the powder mold releasing agent is used, as in the prior art example described above, the mold releasing agent is applied after mold clamping. Therefore, the scatter of the mold releasing agent outside the mold can be reduced. As a result, the powder mold releasing agent can be applied efficiently and the deterioration of the working environment can be prevented. Furthermore, the life of the mold can be increased because the temperature change of the mold in the casting cycle can be reduced.
When die casting is conducted, the inside of the cavity is evacuated in advance to a high vacuum, in some cases, in order to prevent the occurrence of casting porosity resulting from the entrapment of air. Since the air must be purged sufficiently at this time, the degree of vacuum is preferably as high as 20 to 50 Torr.
The inventors of the present invention have confirmed that such a high vacuum need not be established when the powder mold releasing agent is sucked into the cavity. The powder mold releasing agent must be sucked into the cavity and must remain there. If the powder mold releasing agent is sucked at an excessively high degree of vacuum, the amount of the powder mold releasing agent reaching the vacuum apparatus through the cavity increases notwithstanding the requirement that it must be sucked and remain in the cavity. The degree of vacuum required for sucking the powder mold releasing agent into the cavity is 700 to 750 Torr, for example.
The vacuum apparatus for evacuating the inside of the cavity to a high vacuum generally comprises a vacuum tank and a vacuum pump because a vacuum pump having an extremely high capacity must be employed to directly evacuate the cavity by the vacuum pump alone, and the cost of the apparatus increases. Therefore, the vacuum pump and the vacuum tank are combined with each other so that the vacuum pump can gradually reduce the pressure of the vacuum tank. When the degree of vacuum reaches a desired level in the vacuum tank, the vacuum tank is communicated with the cavity to evacuate the inside of the cavity. The following problems arise when such a vacuum apparatus is used to establish both the degree of vacuum necessary for sucking the powder mold releasing agent and the degree of vacuum necessary for air exhaust when the molten charge is ejected.
Once the vacuum tank is communicated with the cavity, the degree of vacuum inside the vacuum tank drops greatly. Therefore, a relatively long time is necessary after the vacuum tank is communicated with the cavity for sucking the powder mold releasing agent and before the degree of vacuum inside the vacuum tank reaches a level necessary for exhausting the cavity. As a result, the casting cycle of the die cast products is long and the productivity drops.
On the other hand when the capacity of the vacuum apparatus is increased (greater capacity of the vacuum tank and greater suction capacity of the vacuum pump) to cope with this problem, the cost of the vacuum apparatus increases drastically.
When a powder molding agent is used as the molding agent, however, the molding agent sucked into the cavity does not necessarily adhere as a whole to the cavity surface, and a part is discharged from the exhaust port of the cavity. When the powder mold releasing agent thus discharged is built up in the vacuum pump and vacuum tank for evacuating the cavity, the desired degree of vacuum cannot be obtained, and trouble in the vacuum pump is more likely to occur.
In the die casting machine of the prior art described above, the powder mold releasing agent is supplied through a sleeve (a feed runner). A plunger for ejecting the molten charge supplied into the cavity is disposed inside this sleeve. The plunger slides inside the sleeve at the time of ejection of the molten charge. Therefore, a lubricant is preferably supplied to insure smooth sliding of the plunger.
However, the lubricant generally has viscosity and when the powder mold releasing agent is supplied, the powder mold releasing agent may be deposited into the sleeve. If the powder mold releasing agent builds up inside the sleeve, the powder molding agent is pushed out into the cavity together with the molten charge when the latter is supplied, and may mix into the die cast product.
It is therefore an object of the present invention to provide a die casting machine and a die casting method that can conduct satisfactorily die casting even when a powder mold releasing agent is used as a mold releasing agent.
It is another objector the present invention to provide a die casting machine and a die casting method that can prevent a casting cycle from being long and can minimize a rise in the cost of a vacuum apparatus even when the inside of a cavity is evacuated for sucking a powder mold releasing agent and even when a vacuum condition is established to prevent air from being entrapped into a molten charge.
A die casting machine according to one embodiment of the present invention comprises a mold, including a fixed mold and a movable mold, forming a cavity when the fixed mold and the movable mold are clamped; evacuation means connected to the cavity through an evacuation passage, for evacuating the inside of the cavity to a predetermined degree of vacuum; switching means disposed in the evacuation passage, for opening and closing the evacuation passage; powder mold releasing agent feeding means for supplying the powder mold releasing agent into the cavity when the switching means is closed and when the inside of the cavity is evacuated to the predetermined degree of vacuum, and applying the powder mold releasing agent to the surface of the cavity; a first filter interposed between the switching means and the evacuation means, and having a filter diameter smaller than at least a mean grain diameter of the powder mold releasing agent; and molten charge feeding means for supplying a molten charge into the cavity after the powder mold releasing agent is applied to the surface of the cavity.
As described above, the first filter having a filter diameter smaller than the mean grain diameter of the powder mold releasing agent is interposed between the switching means and the evacuation means. Consequently, even when the switching means is closed and the evacuation means evacuates the inside of the cavity, the major proportion of the powder mold releasing agent in excess are collected by the first filter. Because the powder mold releasing agent is thus substantially prevented from reaching the evacuation means, a trouble, such as a failure to reach the desired degree of vacuum, can be prevented.
A die casting method for conducting die casting by clamping a fixed mold and a movable mold to define a cavity and supplying a molten charge into the cavity from a sleeve according to one embodiment of the present invention comprises a first step of supplying a powder mold releasing agent into the cavity through the sleeve; a second step of supplying a lubricant for a plunger sliding inside the sleeve, after the first step; a third step of supplying the molten charge into the sleeve and ejecting the molten charge by the plunger into the cavity after the second step; and a fourth step of withdrawing a metal mold product solidified inside the cavity.
In this method, as the powder mold releasing agent is supplied into the cavity through the sleeve when the inside of the sleeve is in a dry condition, the powder mold releasing agent is not built up inside the sleeve. The lubricant is supplied into the sleeve for the plunger, and smoothly slides inside the sleeve, after the powder mold releasing agent is supplied.
A die casting machine according to another embodiment of the present invention comprises a mold, including a fixed mold and a movable mold, and forms a cavity when the fixed mold and the movable mold are clamped; powder mold releasing agent feeding means for supplying a powder mold releasing agent into the cavity; first evacuation means connected to an exhaust port of the cavity through a first evacuation passage, for evacuating the inside of the cavity to a first predetermined degree of vacuum and sucking the powder mold releasing agent supplied from the powder mold releasing agent feeding means into the cavity; second evacuation means connected to the exhaust port of the cavity through a second evacuation passage, for evacuating the inside of the cavity to a second predetermined degree of vacuum higher than the first degree of vacuum after the powder mold releasing agent is applied to the surface of the cavity; and molten charge feeding means for supplying the molten charge into the cavity when the second evacuation means evacuates the cavity to the second predetermined degree of vacuum.
The first degree of vacuum for sucking the powder mold releasing agent is established by the first evacuation means. The second degree of vacuum for purging the air inside the cavity to prevent the air from being entrapped by the molten charge is established by the second evacuation means. The first and second degrees of vacuum are attained by the first and second evacuation means that are provided independently of each other. Therefore, a waiting time for acquiring the predetermined degree of vacuum is not needed. In consequence, the casting cycle can be prevented from becoming long.
Further, the present invention takes the difference between the first degree of vacuum, for sucking the powder mold releasing agent, and the second degree of vacuum, for purging the air inside the cavity to prevent air from being entrapped into the molten charge, into specific consideration. When the powder mold releasing agent is sucked into the cavity, a high degree of vacuum required for achieving the high vacuum state for preventing entrapment of air into the molten charge is not necessary. In this case, the first evacuation means is so set as to attain a lower degree of vacuum than the second evacuation means. Therefore, although two independent evacuation means are provided, the increase in the cost can be restricted.
A die casting method for conducting die casting by supplying a molten charge from a sleeve into a cavity that is formed when a fixed mold and a movable mold are clamped, according to another embodiment of the present invention, comprises a first step of evacuating the inside of the cavity to a first predetermined degree of vacuum by first evacuation means; a second step of sucking a powder mold releasing agent supplied through the sleeve into the cavity by utilizing the first degree of vacuum; a third step of evacuating the inside of the cavity to a second predetermined degree of vacuum higher than the first predetermined degree of vacuum, by second evacuation means disposed separately from the first evacuation means; a fourth step of ejecting the molten charge, that is supplied into the sleeve, into the cavity evacuated to the second degree of vacuum, by a plunger sliding inside the sleeve; and a fifth step of withdrawing a metal mold product solidified inside the cavity.
The die casting method of the present invention described above can prevent the casting cycle from becoming long while the increase of the cost is restricted.